The present invention relates generally to static mixers, and more particularly, to an extensional flow mixer followed by helical type mixing elements, preferably also followed by of high-shear, high-pressure drop static mixing elements, that mixes two or more fluid streams flowing in a pipe.
It is often desirable to mix fluids having varied viscosities in a pipe. In a turbulent flow, mixing occurs more quickly due to induced turbulence. In a laminar flow, mixing of fluid streams is more difficult. In solution polymerization, for example, it is often desirable to mix a relatively high viscosity bulk stream, such as a polymer solution, with a relatively low viscosity liquid additive stream. Liquid additives, catalysts, liquid monomers and solvents are typically added to polymer solution to achieve other polymer products.
However, because of the high shear forces necessary to promote mixing, the high viscosity bulk stream and the low viscosity additive stream may remain essentially segregated, resulting in low rates of additive stream incorporation into the bulk stream. In a laminar flow, mixing occurs by diffusion of one stream into another, which typically is a slow process. The slow diffusion is unacceptable when a quicker mixing time is necessary for dispersion. Frequently, when the additive stream is injected into the bulk stream, the additive stream will remain substantially intact and tunnel through the bulk stream without significant interfacial mixing of the streams. This low mixing rate is due in part to the low surface area contact between the bulk stream and the additive stream. To combat such a result, it is advantageous to deform the additive stream from the cylindrical shape the additive stream initially has, to a relatively flat sheet having more surface area. It is found that deforming the additive stream by increasing its aspect ratio, the ratio of its width to its height, increases its surface area and therefore its potential interfacial mixing area. The increase in surface area also facilitates the strategy of cutting, dividing and recombining the streams in traditional static mixers. The distribution of the additive stream as a thin sheet also increases the mixing efficiency of the static mixing elements, if any, following the extensional flow mixer.
Several types of structures are known to promote mixing of a bulk stream with an additive stream, including baffle structures and shear mixers. U.S. Pat. No. 4,808,007, issued to King, discloses a dual viscosity mixer which introduces an additive stream to a bulk stream through an entry port within the mixer to create an elongated flat plane of the additive stream.
Several problems have been encountered in the field with this and other mixing structures, however. For example, in polymerization applications, polymer build-up has been observed at the contact points between the additive stream injector and the bulk stream polymer. This build-up often occurs when the additive stream is injected from within the static mixer. The polymer build-up problem compounds itself until eventually there is plugging or complete closure of the additive injector, leading to flow maldistribution in the static mixer.
Additionally, when an additive stream, such as a catalyst, contacts a baffle or other solid contact surface or wall, a wetting of the surface with the catalyst occurs, thereby decreasing the overall mixing efficiency of the catalyst with the bulk stream.
In those mixers where there are severe angular regions or step-like features, the bulk stream and the additive stream, while flowing out of such features, may develop recirculation zones and eddy currents, which decreases the overall mixing efficiency of the mixer.
Another problem is the loss of fluid pressure as the streams pass the mixer. Other dual viscosity mixers available have a relatively high pressure drop, as the streams lose fluid pressure between entering and exiting the mixer.
International Publication No. WO 00/21650 discloses an extensional flow mixer for mixing a bulk stream with an additive stream. Two extensional mixers may be arranged in series with a gap of approximately the diameter of the flow conductor to promote additional mixing capabilities. The extensional mixer may be used in laminar, transition or turbulent flow conditions.
While the prior art discloses mixers that mix bulk streams with additive streams, there exists a need for a mixing system that improves the degree of mixing of the bulk stream and the additive stream by increasing the dispersion of the additive stream within the bulk stream, which further increases the interfacial area between the two streams.